Field of the Invention
The present application discloses a method for carbonation with CO2, which can be applied as example in the sugar refining.
Related Art
The word “sugar” is currently used for the chemical sucrose. Sucrose is a member of a group of substances generally known as sugars, which contain up to ten monosaccharide units, wherein monosaccharides are carbohydrates that cannot be further hydrolyzed. All carbohydrates are compounds built up from the elements carbon, hydrogen and oxygen. All sugars are crystalline, water soluble and sweet tasting.
Sucrose has the chemical formula C12H22O11. It may be converted by acid or enzymatic hydrolysis into a mixture of two sugars, glucose and fructose, each with the formula C6H12O6, through the following general reaction:C12H22O11H2O→C6H12O6+C6H12O6 
In sugar refining, glucose and fructose are regarded as impurities due to the difficulty of crystallizing them from the solution. Due to this, strict control of pH must be maintained to avoid loss of sucrose during refining through chemical hydrolysis to glucose and fructose.
Sucrose is purified from raw sugar, which is about 97.5% sucrose, in a four step process comprising the following steps:                affination—dissolving off some surface impurities;        carbonation—removing further impurities that precipitate from solution with calcium carbonate;        char filtration—removing further impurities with activated carbon;        crystallization—using a heat/vacuum process to produce sugar crystals.        
In carbonation, milk of lime, which is calcium hydroxide, is added to the heated liquor, and boiler flue gas, containing CO2, is bubbled through the mixture. The chemical reactionCa(OH)2+CO2→CaCO3+H2Ooccurs under controlled conditions and as the calcium carbonate precipitate is formed, it precipitates a number of impurities, including multivalent anions such as phosphate, sulfate and oxalate, and large organic molecules such as proteins and pectins which aggregate in the presence of multivalent cations, removing them from the sugar syrup. The carbonation process is carried out in two stages, namely, two stages of carbonation with flue gases containing CO2 in tanks by bubbling the flue gases in the liquor to obtain an optimum quality precipitate for filtration, i.e. a suitable size and distribution of precipitate particles. The temperature of liquor shall be maintained between 70° C. and 90° C. by injecting steam in an exchanger built in each tank.
Eighty to ninety percent of precipitation is sought in the first stage of carbonation. The second stage is controlled by the measurement of the pH of the solution which is important throughout the process and ensures complete precipitation of the lime. The total reaction time is around 1 to 1.5 h at around 80° C.
The pH of liquors is of considerable importance. Below pH 7, sucrose is hydrolyzed to glucose and fructose, while above pH 9, alkali destruction of sugars occurs and coloured components are formed.
The calcium carbonate precipitate, including the impurities, is removed in a pressure filtration step using a filter cloth as supporting media and utilizing the calcium carbonate as a filter aid. The filter mud is later subjected to water washing to remove sugar residual and this mud is treated as a waste material. Water containing sugar recovered by washing the mud is used for dissolving the raw sugar at an earlier stage.
This operation of carbonation can be performed by flue gases containing CO2 from the sugar mill boilers. By doing this, the calcium hydroxide added to the sugar liquor precipitates as CaCO3 and reduces the impurities in the sugar syrup prior to crystallization. Yet there is a very important drawback: the CO2 contained in the flue gases depends on the quantity and quality of the fuel being burned. Additionally the flue gases must be washed in a scrubber system to remove solid particles, SOx and NOx and this system produces liquid effluents that must be treated externally. Furthermore the flue gas is compressed using liquid ring compressors that use a high amount of electricity. The most common fuel used in the boilers, used to be fuel oil which produced flue gases with a content of ˜12% CO2. Yet, in present times due to environmental concerns, fuel oil is increasingly being substituted for natural gas which produces a flue gas with 6% CO2. In some cases, sugar mills are stopping the boilers and installing combined cycle systems which have the advantage of producing electricity as well as steam but produce a flue gas with 2˜3% CO2. In these two events the quantity of CO2 generated is not sufficient for the carbonation process and mills are known to partially change a part of the natural gas used by fuel oil only to increase the CO2 content of the flue gas.
The document U.S. Pat. No. 6,176,935 discloses a system where flue gases from a boiler are first scrubbed and then passed through a gas separation membrane module. After the gas has passed through the membrane module, the concentration of carbon dioxide in the stream is increased to about 20% in volume. This stream is then injected into a reactor containing raw sugar, to perform the step of carbonation, and thus to remove most of the coloring matter from the raw sugar. However, this document does not disclose the use of a static or dynamic mixer to react with the CO2 in a carbonation step.
The document EP0635578 discloses a method of refining brown sugar that comprises a step of carbonation and/or phosphatation of said brown sugar.
However, this document does not disclose the use of a static or dynamic mixer to react with the CO2 in a carbonation step.
The document GB1239407 discloses a process for producing aragonite comprising the reacting carbon dioxide with calcium hydroxide dissolving in a sucrose solution at a temperature from 60° C. to 90° C. in the absence of crystal poisons in amounts preventing the formation of said aragonite. However, this document does not disclose the use of a static or dynamic mixer to react with the CO2 in a carbonation step.
The document GB1106276 discloses a method of refining a raw sugar juice comprising initial defecation-saturation with simultaneous addition of some of the total required quantity of lime and carbon dioxide in a low alkaline pH range between 8 and 10. However, this document does not disclose the use of a static or dynamic mixer to react with the CO2 used in a carbonation step.